Data cable is a collection of filaments laid together so that the assembly can be handled conveniently. In the present context, the filaments may be wires, insulated wires, pairs, coaxial tubes, optical fibers, etc. The data cable preferably has sufficient strength and flexibility for its purpose. A common way to achieve this is to twist the filaments together to form a collection of helices. That not only forms a compact cable in cross-section, but also gives flexibility, so that when the cable is bent, the portion on the outside of the bend draws the necessary extra length of filaments from the inside of the bend. That suggests that the cable should not be so compacted that the filaments cannot move relative to each other. On the other hand, too loose a cable will easily deform or flatten when bent or compressed.
Unshielded twisted pair (UTP) cabling is the most common cable used in computer networking. It is a variant of twisted pair cabling. UTP cables are often called “Ethernet cables”, the most common data networking standard that utilizes UTP cables, although not the most reliable. In contrast to FTP (foil twisted pair) and STP (shielded twisted pair) cabling, UTP cable is not surrounded by any shielding. UTP is the primary wire type for telephone usage and is very common for computer networking, especially in patch cables or temporary network connections due to the high flexibility of the cables.
STP cable comprises a number of shielded twisted pairs within an overall screen and sheath. The benefits of STP cabling versus UTP cabling is a debate gaining momentum as data transmission speed increases. If, for example, CAT 7 cable using UTP is incorrectly installed, its performance could be worse than CAT 5 cable. And with STP, ground loops, current flowing along a shield between grounds at different potentials, can inject noise into the wires that the shields are intended to protect.
One factor contributing to lack of progress in the adoption of CAT 7 is the confusion caused by the manufacturer-specific nature of actual CAT 6 installations. In practice, CAT 6 cables, connectors, patch panels, and related products cannot be mixed with those from another manufacturer without degrading system performance. One effect of the subtle differences among components is to cause impedance mismatches that generate reflections and affect return loss.
Alien crosstalk (AXT) is electromagnetic noise that can occur in a cable that runs alongside one or more other signal-carrying cables. The term “alien” arises from the fact that this form of crosstalk occurs between different cables in a group or bundle, rather than between individual wires or circuits within a single cable. Alien crosstalk can be particularly troublesome because, unlike the simple crosstalk caused by a single interfering signal, it cannot be eliminated by phase cancellation. Alien crosstalk arises from multiple signals, and includes mixing products in which phantom signals at innumerable sum and difference frequencies blend with the originating signals. The result is a “hash” of electromagnetic noise that is too complex to be dealt with by phase-cancellation measures. Because it resembles noise rather than signals, alien crosstalk degrades the performance of a communications system by reducing the signal-to-noise ratio (S/N).
Alien crosstalk can be minimized or eliminated by avoiding configurations in which cables are bundled together or run parallel to one another in close proximity. If cables must be run parallel to each other, each cable can be surrounded by a grounded metal braid (STP or electromagnetic shield) to prevent electromagnetic fields from entering or leaving the cable. This in effect isolates the cables from one another. However, it is an expensive solution and it can also increase cable loss per unit length.